The present invention relates to turbochargers and, more particularly, to turbochargers having a centrifugal compressor that includes a pair of impellers arranged in a back-to-back configuration such that air enters one impeller in a first axial direction and air enters the other impeller in a second axial direction opposite to the first axial direction.
Conventionally turbocharged internal combustion engines employ a turbocharger having a single turbine wheel that receives exhaust gas from the engine and is driven by the exhaust gas to rotate a centrifugal compressor wheel comprising a single impeller. The impeller compresses air and delivers the air to the engine intake system, where the air is mixed with fuel and supplied to the engine cylinders for combustion. Turbocharging allows the engine to achieve higher power output than an equivalent non-turbocharged engine.
Various trends in engine design, particularly with respect to diesel engines, have made it increasingly difficult to achieve adequate turbocharger performance using a conventional single turbocharger as describe above. Such trends include increasing requirements for engine power, as well as government regulations reducing the allowable limits of NOx and particulate emissions. It has been found that a single compressor is not capable of meeting the pressure ratio and flow range requirements of some state-of-the-art engine systems.
Recognition of this problem has led to the development of various types of turbocharger systems that employ multiple compressor stages. For example, serially arranged turbochargers have been developed, in which the turbines of two turbochargers are arranged in series and the compressors are arranged in series. While such series turbochargers can achieve performance improvements over single turbochargers, they are expensive, and are bulky and hence difficult to incorporate into engine compartments that are already cramped for space.
An innovative solution to this problem is disclosed in commonly assigned U.S. Pat. No. 6,948,314 to Arnold et al. The '314 patent describes a single turbocharger having a compressor wheel comprising two impellers mounted on the same shaft and arranged in a back-to-back configuration. Each impeller has its own air inlet, and the air pressurized by each impeller is discharged into a common volute. A movable flow-control member is disposed between the compressor wheel and the volute and is movable between a first position in which both impellers discharge into the volute, and a second position in which the discharge flow path of one of the impellers is effectively shut off so that only the other impeller discharges to the volute. This compressor arrangement allows the compressor flow range to be extended, and allows the compressor wheel diameter to be reduced, relative to a conventional single compressor. The diameter reduction leads to a reduction in rotor inertia, thereby improving transient response of the turbocharger. The arrangement also facilitates matching between the compressor and turbine.